Canvasses
On TV Bob always used a 18”x24” double primed canvas. He strongly recommended stretched canvas over a frame, and NOT canvas boards. (He felt the boards sucked up the oil in the paint.
Most canvas you buy today state they are triple primed. However on the less expensive canvases, you will probably get better results if you prime it again with another coat of White Gesso. Use a cheap foam brush to do it.
The Bob Ross canvas (available from this web site) is very high quality, and works well without additional priming. One helpful thing Bob did when he made the specification for the Bob Ross Canvas is that he had it primed in a very light gray so that you can see the coverage of your liquid white when you start a new painting.
You can use any size canvas you like. On TV Bob painted with an 18x24 canvas. If you want to start painting with smaller canvases, 12x16 is a good size.